Today wireless communication systems such as cellular telephone and private mobile radio communication systems typically provide for radio telecommunication links to be arranged between a number of base stations and a plurality of subscriber units. Here and in accordance with telecommunication parlance, a subscriber equipment is referred to as a user equipment (UE) while a base station may be referred to as a NodeB or eNodeB.
Some examples of UEs include cellular phones or radios, personal digital assistants (PDAs), MP-3 players, wireless video units, Internet units, computers and other electronic devices capable of mobile communication.
In a wireless communication system, each base station has associated with it a particular geographical coverage area (cell). The cell is defined by a particular range within which the base station can maintain acceptable communications with UEs. Often several cells combine to produce an extensive coverage area and commonly UEs move within as well as between various cells, which includes movement between cells of different service providers. In doing so, UEs encounter varying radio propagation environments in different base station cells.
As radio transmission conditions change for a communication link between a UE and a base station, for example in dependence of how the UE is moved within or between different cells of various base stations, the power requirements for the radio transmission varies if a certain radio transmission quality shall be maintained. Here, the communication link from the base station to the UE is referred to as the downlink channel while the communication link from the UE to the base station is referred to as the uplink channel.
When determining uplink power requirements for a UE, it is possible to examine received downlink signals and extract at least some of the information required to determine the proper uplink transmission power since there is typically a proportionality between the received signal power and the power necessary to use for transmitting a signal. If the transmission power of a signal received at the UE is known, or if there is some way to estimate the transmission power, the necessary transmission power to reply to that transmission can be estimated.
However, for certain types of communication systems, such as systems that use time-division multiplexing to separate downlink and uplink signals, transmission power estimates may be very inaccurate. This is because the accuracy of signal level measurements is affected by the duration of the measurement and the elapsed time since the measurement was last taken. If the duration of the measurement is too short, variations in the measured signal level adversely affect the measurement. If the elapsed time is too long, the measurement becomes less representative of the present state of the signal level due to interim changes in the signal.
For example, in the case of a UE in a time division duplex (TDD) system, the power of the transmission may vary greatly between timeslots as well as within a timeslot. This power variation is affected by various factors such as the physical movement of the UE and by the environment where the base station and the UE is located.
More specifically, if a UE is moving at a moderate speed it is possible to lose significant signal power in several tens of milliseconds. If a radio frame is ten milliseconds long significant signal variations can occur even within the frame. Therefore it is important that path loss measurements (i.e. measurements of how much power of the signal is lost during transmission from a UE to a base station or vice versa) are applied as fast as practical since an instantaneous measurement of signal characteristics can quickly lose its validity.
Furthermore, path loss estimation between a UE and a base stations is an important parameter for several other radio resource management functions, such as handover, link adaptation, scheduling and inter-cell interference co-ordination.
Today methods exist for determining path loss in telecommunication systems employing for example TDD, which is the case for e.g. a Universal Mobile Telecommunication System (UMTS) or a telecommunication system implemented in accordance with the specifications of Third Generation Partnership Project Long Term Evolution release 8 (3PP LTE release 8). Both of these systems use for path loss measurements a downlink pilot signal that is transmitted to all of the UEs.
In detail, for TDD systems used today a UE measures the received signal power of a serving base station's common control physical channel (P-CCPCH) or other pilot channel, and calculates the path loss (for the downlink pilot channel) between the base station and the UE. This has the advantage of providing an accurate path loss measurement immediately after the pilot signal is transmitted. The path loss measurement based on the received signal power is thereafter used to determine the transmit power for uplink physical channels. Examples of such uplink physical channels include the physical uplink shared channel (PUSCH), the dedicated physical channel (DPCH), the physical random access channel (PRACH) and the high speed shared information channel (HS-SICH). As mentioned before, the measured path loss is also transmitted to the base station as an input value in various radio resource management functions.
Even though the example above elucidates problems related to systems that use TDD, similar problem are present in systems that use Frequency Division Duplexing (FDD) where path loss measurements, accordingly, are just as important. In the case of FDD, in addition to the delay between a measurement and its application, there is also a frequency domain separation between uplink and downlink that adds to the inaccuracy when a path loss estimate for the downlink is used to set appropriate transmit power in the uplink.
Hence, in order to obtain proper uplink power control and radio resource management, path loss measurements must be regularly transmitted from UEs to base stations, which is typically done with RRC (Radio Resource Control) reports when certain pre-configured threshold values and time triggering conditions are fulfilled. This means that radio resources are used every time a path loss estimation is reported, which reduces the capacity for transmitting value-adding data which includes e.g. operators' services and various user data transported over the telecommunication system.